1. Field of the Invention
The present invention relates to a device and method for distributing a liquid sample onto a test surface. More particularly, the present invention relates to a reagent format which draws and distributes fluid over a testing area by capillary action.
2. Description of Related Art
Many different designs of test apparatus which involve chemical analysis of liquids such as water, milk, as well as biological fluids including blood and urine are known in the art. Some of these testing apparatus are suitable for liquid analysis, wherein there is required the addition of a liquid reagent for analysis of a substance termed an "analyte". The reagent, upon contacting a liquid test sample containing the analyte, affects formation of a colored material or other detectable change in response to the presence of the analyte. Other systems depend on a dry system, such as pH papers and the like, where the paper or other highly absorbent carrier is impregnated with a material which is chemically reactive or responsive when placed in contact with the liquid containing the analyte. The response or reaction generates a color change or other type of detectable change. Depending to a great extent upon the selection of responsive material, the change is usually qualitative or at best semi-quantitative.
For diagnostic chemical analysis wherein the testing of biological fluids such as blood, plasma, urine and the like are utilized, it is preferable to produce highly quantitative results rapidly, conveniently and with assurance of accurate results. Also, it is desirable to have precise control and monitoring of the liquid specimen that is being subjected to the test. The ability to precisely control and monitor the liquid specimen is particularly important in tests involving machine reading of the reaction. In analysis involving machine reading it is important to have the reading machine begin its analysis at the appropriate time. In addition, when relying on machine readings, it is important that a calibrated amount of the test specimen be exposed to the test substrate so that the proper reaction will take place and any interference with optical detection or other detection of color changes is avoided.
A variety of devices and methods are relied upon in presenting the test liquid containing the analyte to the surface of the substrate. Many dry chemistry reagents have the liquid test sample placed upon the test substrate surface and then spread by the user. The application of the liquid test sample volume and the spreading thereof is user variable and influences the performance of the reagent. In addition, such user contact is undesirable from the standpoint of increasing the chance of user contamination--especially when dealing with body fluids such as blood or plasma.
In U.S. Pat. No. 4,776,904 there appears discussion focusing on reagent films which represent one form of the test substrates described above.
Additional testing devices utilizing dry chemistry reagents include devices such as those disclosed in U.S. Pat. Nos. 4,647,430; 3,814,668 and 3,215,855 where the testing device is dipped into the fluid. These devices, however, are not suitable for many test requirements due to the drawbacks of requiring large volumes of liquid test samples as well as the increased likelihood of contamination brought about by the need for wiping off excess fluid to avoid dripping.
The ONE TOUCH (.TM.) by Lifescan presents a reagent format wherein a drop of blood is placed on an exposed surface of a reagent film and the results are viewed from the other side of the film. The sample is introduced by having the user transfer the sample from the fluid sample source to the exposed film surface. The amount and distribution of the fluid sample is user variable and thus difficult to control.
A variety of devices and methods have been developed for transporting liquid in a controlled and predetermined flow pattern. Many of these items have been concerned with uncontrolled and undirected capillary flow of the liquid across surfaces. Some problems that have been encountered with uncontrolled flow include formation of trapped air pockets and incomplete wetting of certain portions of the surface. Air pockets create problems when the test device is examined through a microscope or by way of automatic methods because the examination of the liquid and/or wetted surfaces results in different test data being collected. The examinations involving automated systems are based on a presumption of the presence of the liquid in the scanning area and therefore the absence of the liquid in the relevant scanning area will throw off the value of the reading and will give an unreliable result. The problem of air pockets is a common occurrence particularly when dealing with configurations which have sharp corners and synthetic resin surfaces which are generally hydrophobic.
A variety of different types of liquid transport devices have been developed in the prior art including that shown in U.S. Pat. No. 4,761,381 to Blatt et al. Also, in Columbus, U.S. Pat. No. 4,233,029 there is described a device containing a means for directing capillary flow along predetermined paths by use of grooves in the opposed surfaces of a capillary chamber.
Another configuration for the transport of a liquid test specimen is shown in Columbus, U.S. Pat. No. 4,254,083, which provides for an exterior drop receiving surface containing a particular opening configuration which is intended to facilitate the centering of the drop.
Buissiere et al., U.S. Pat. No. 3,690,836, describes a device consisting of a capillary space between two plastic sheets which are sealed in a continuous perimeter and which enclose an uncompressed absorbent material which fills the capillary space. At least one opening at the top sheet provides for access to the reaction chamber.
A liquid transport device which provides for diversion of capillary flow into a second zone is shown in Columbus, U.S. Pat. No. 4,473,457. The device has two pathways for flow of the specimen and permits the introduction of two different specimens through two apertures. The two liquids then will flow towards and into a common area. The configuration of the structure of Columbus permits potentiometric determinations to be made. See also Columbus, U.S. Pat. No. 4,302,313, which shows a device suitable for potentiometric analysis of liquid ions. Special grooved surfaces under the member 36 are said to control capillary flow.
Another device is shown by Columbus, U.S. Pat. No. 4,271,119, which has a downstream diverting aperture in a wall member of a first capillary zone which provides capillary flow into a second capillary zone extending from that wall member.
The aforementioned liquid transport devices rely on the placement of a fluid test sample within an access aperture before the capillary action can induce fluid flow within the transport devices.
Again, the reliance on user insertion of the fluid sample into the access aperture means that there will be a subjective determination as to how much fluid to insert into the access aperture. Such subjective determination as to the appropriate volume of fluid is often too high thus necessitating an overflow device or subsequent wiping off of the device surface. Moreover, when the inserted volume of fluid is insufficient, repeated insertions often result in the last insert exceeding the desired volume which again leads to an overflow. Further, the need for an overflow chamber and/or wiping off of the surface is an indication that the volume of fluid test material is not being efficiently utilized.
Additionally, even with the liquid transport systems, evaporative cooling presents a problem in precision testing. There is also present in the prior art the problem of not obtaining a sufficient amount of sample on the test substrate due to difficulty in determining or detecting when the test substrate is sufficiently covered.